A. Technical Field
The present invention relates to a LED driver. More particularly, certain embodiments of the present invention relate to using an NPN bipolar transistor (BJT) to drive a LED string to minimize high voltage driving voltage pins and reduce the risk of short circuit.
B. Background of the Invention
Semiconductor-based solid-state lighting (SSL), until recently associated mainly with simple indicator lamps in electronics and toys, has become as bright and more efficient than other lighting technologies. In particular, the enormous technology improvements have been achieved on light emitting diodes (LEDs) over the past years. LEDs have been available for various wavelengths and suitable for white illumination. Lifetime of LEDs is also extended to more than 100 thousand hours, and can work at input powers up to many watts.
LEDs are connected in series as a LED string for use in lighting applications. Conventional LEDs used as indicator lamps operate with low voltage and current (e.g. 1.5V, 10 mA) and a constant direct current (DC) supply is sufficient to drive these indictor LEDs. However, each power LED in the LED string used for backlighting or illumination requires a nominal current anywhere in the range of 35-1400 mA, a forward voltage drop of 3V and large manufacturing tolerances. The LED string has to be driven with power electronics to provide controlled LED current and avoid power consumption by series parasitic resistors.
In order to avoid losses in the series resistors, the LED strings are powered by switched mode power supplies and linear regulators are normally used to stabilize the LED output current when more than one string is present. The standard buck, boost and buck-boost DC/DC converter topologies may be used to provide the supply voltage needed by the LEDs.
FIG. 1 illustrates a standard LED lighting system 100 comprising LED driver electronics 102 and LED strings 104, which are implemented on two separate electronic boards. The driver electronics board 102 and the LED string board 104 are further packaged into a system using cables. The two illustrated LED strings on the LED string board 104 share a common power supply 120 and have separate pins 122 and 124 to drain the current. The core of the LED driver electronic board 102 is a LED driver controller 106 which receives appropriate power supplies and digital controls and generates analog voltages needed to bias LED driver transistors 108 and 110 in a boost converter configuration. The LED driver transistors 108 and 110 are two NPN bipolar junction transistors, each driving a separate LED string. Although most driver electronics are integrated on the driver controller chip 106, discrete components are preferred for the LED driver transistors 108 and 110 due to their capability of handling larger current. The driver transistors 108 and 110 are integrated with the LED driver electronic board 102 with connections to three pins (collector, base and emitter) of the driver integrated circuit (IC) 106. The current drains of the LED string board 104 are connected with the collectors 122 and 124 of the driver transistors 108 and 110. The current provided by the driver transistors 108 and 110 ranges from 30 mA to 350 mA to meet the requirements for various applications, such as notebook computers, LCD monitors or flat screen televisions.
For use in backlighting displays, the standard LED lighting system 100 has to address several concerns. The LED drive current should be monitored and controlled in the LED driver electronics 102 as needed. System efficiency is optimized only if optimal boost output voltage is applied to minimize the voltage on the collectors of the driver transistors 108 and 110. Pin reduction of the custom driver controller chip 106 is always desirable. In particular, reduction of the analog pins to the driver transistors is highly preferred because it can greatly reduce the cost of the chip 106. In this boost converter configuration, both the high-end supply 120 and low-end current sinks 122 and 124 are at high analog voltage levels and the risk of shorting them to ground always exist as far as embedded system integration between boards or chips is involved.